How do I get the IP from a URL?
Yes, you can use a scripted lookup, which will do a reverse DNS check on a given URL.
Please check the example here: http://docs.splunk.com/Documentation/Splunk/5.0.4/Knowledge/Addfieldsfromexternaldatasources#Example...
This is for an older version of Splunk, but the script is still shipped, and can be used as described.
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Use rex command in search, use sedcmd in props. Did you want to make the ip address an indexed field or search time extraction?
|rex field=urlField "[regex_to_capture_ip]"
|rex field=urlField "(?<IP>(\d+\.\d+\.\d+\.\d+))"
To use DNS in order to resolve a CNAME like yahoo.com to its A or AAAA records, please use the answer posed by DMohn.
If you have an IP address in a URL field, please use my answer.